The radial-velocity wobble induced by a small exoplanet is much smaller than stellar microvariability, therefore effects of the latter must somehow be calibrated and corrected for. Numerous authors have studied empirical correlations between various stellar activity indices and corresponding excursions in apparent radial velocity [1]}, [2]}, [3]}, [4]}, [5]}, [6]}, [7]}, [8]}, [9]}, [10]}, [11]}, [12]}, [13]}, [14]}, [15]}, [16]}, [17]}, [18]}, [19]}, [20]}.
